Precision radiation attenuator

ABSTRACT

Attenuation of electromagnetic radiation from the ultraviolet through the infrared region of the electromagnetic spectrum is achieved in an extremely precise manner through the use of a pair of reflecting spheres of precise roundness, arranged so that radiation polarization effects are small. One of the spheres is rotated to further reduce stray radiation, and has a portion which is blackened to further reduce the radiation.

United States Patent Inventors App]. No. Filed Patented AssigneePRECISION RADIATION ATTENUATOR [56] References Cited UNITED STATESPATENTS 3,083,612 4/1963 Miller 350/293 X 3,313,154 4/1967 Bruce 331/945UX 3,508,056 4/1970 Fricke 250/833 H Primary Examiner-David SchonbergAssistant Examiner-John W. Leonard AttorneysHarry A. Herbert, Jr. andRuth G. Codier 1 claimsnrawing Figs ABSTRACT: Attenuationofelectromagnetic radiation from 11.8. CI 350/294, the ultraviolet throughthe infrared region of the electromag- 250/833 H,250/236, 350/7,350/289, 356/43, netic spectrum is achieved in an extremely precisemanner 356/216 through the use of a pair of reflecting spheres ofprecise Int. Cl G02b 5/10 roundness, arranged so that radiationpolarization effects are Field of Search 350/ 1,7, small. One of thespheres is rotated to further reduce stray 289,293,294;250/83.3 H,236,217 R; 356/216, radiation, and has a portion which is blackened tofurther 43; 331/945 reduce the radiation.

I: I/ll/l/l/l/III/ll/IIl/l/llll/ I)? /II/Y]}AI////////// ll/Il/l/lr/ IdBACKGROUND OF THE INVENTION The invention relates to a precisionradiation attenuator, and more particularly to a device for receivingradiation, spreading it out, preventing it from polarizing, and gettingmeasurable attenuation of the initial radiation.

The need for new and more precise methods of controlling and dispersingor attenuating radiation from all kinds of sources is well known. Mostof the old methods require large unwieldy configurations, giving rise toerrors in the reduced exiting radiation.

Some of the old methods use the principle of reflection from sphericalsurfaces, generally convex mirrors, or the convex surface of a lenswhich has been metal-coated. Still others use the principle of diffusionand attenuation by filtering.

These methods and others now available, do not achieve the control ofpolarization effects, stray radiation effects and sufficient precisionin the spherical surfaces used, and most of them require complicated andcumbersome designs. This gives rise, as above stated, to increasedexcitation, which is the opposite of the result which is desired.

Actually, no convenient standard of spectral flux existed until therecent establishment of a standard of spectral radiance in the form of atungsten strip lamp by direct comparisons with the radiances ofblackbodies. This standard, however, yields irradiances for reasonableaperture sizes and distances that are many times too high for certaincalibration work requiring photomultipliers, scintillators, or otherhigh sensitivity detectors for measuring low irradiances such as thosefrom the air-glow, or from weak phosphorescent, fluorescent, orbiochemical sources. Although miniature diaphragms might be employed,the interferencefringe effects resulting therefrom, together with thenecessity of working at great distances from this standard, rule out itsuse in this area without the use of auxiliary optics.

SUMMARY OF THE INVENTION The object of the invention is the provision ofa device and method for precision attenuation of electromagneticradiation from ultraviolet through the infrared region of theelectromagnetic spectrum. The method has an extremely high degree ofaccuracy and a capacity for attenuation to a very low level irradiance(of the order of watts).

The device has potential as an instrument for standards of extremely lowirradiance, and has potential also as a laser power meter.

These effects are achieved through the use of reflecting spheres ofprecise curvature so arranged that polarization effects are very small.These spheres are rotated at precisely designed speeds, each reducingpolarization and stray radiation effects.

ln summary, the features believed to be new are as follows:

1. Use of precision ground reflecting spherical bodies.

2. Precision interchangeability of spheres and flat reflecting surfaces.

3. Reduction of polarization effects with a small easily handledconfiguration for laboratory use.

4. Reduction of internal stray radiation effects by effecting rotationof one of the reflecting spherical bodies, and providing a blackabsorbing surface on the rotating sphere.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawing.

DESCRIPTION OF THE DRAWING FIG. 1 is a top plan view of the precisionradiation attenuator according to the invention;

FIG. 2 is a vertical longitudinal cross-sectional view; and

FIG. 3 is an end view of the device.

DESCRIPTION OF A PREFERRED EMBODIMENT The radiant flux indicated at 10in FIG. 1 enters an aperture 12 in an enclosure 14. The aperture 12becomes in effect a source of radiation which impinges on a first sphere16. This sphere 16 is interchangeable with elements of existing devices,such as flat metallic reflectors. The radiation A entering the opening12 and striking the sphere 16, is spread out, some of it striking asecond sphere 18. The sphere 18 is mounted for rotation, and has areflecting portion 20 and a nonreflecting blackened radiation absorbingportion 22.

The radiation 8 which reaches the sphere 18 from the sphere 16 isfurther dispersed and partially absorbed, a portion of it indicated at Cfinally reaching the exit aperture 24 and emerging as greatly reducedradiation of known quantity.

Means, such as a motor 30, plug 32 and standard gear train 34, and/orpulley 36 are provided for the rotation of the sphere 18. These arestandard elements to which the invention is not limited, and which formno part of it.

The degree of attenuation is known from quantitative relationshipbetween the radii of curvature of the spheres, the spectral reflectanceof their surfaces, and the distances and angles between them. Therotation of the second sphere 18 allows the use of synchronous detectiontechniques and thus eliminates stray radiation from the walls of theenclosure that may occur from the first sphere 16. Flat minors may beinterchanged with the spheres 16 and 18 in combination or alone to givefour combinations of attenuation. Angles of in cidence and reflection ofradiation with the spheres have been selected to minimize polarizationeffects. The accuracy of the spherical surfaces is accurately known tobe of the order of 1 part in 10,000. Reflectance and distances can beknown accurately to l part in 1,000, depending on the metal surfacesused.

Precision has been improved by the use of precision ground spheres, lowangles of incidence and reflection, and by dropping the radiationinternally in the enclosure.

An alternate method of construction would be to add successivereflecting surfaces, either spherical or flat, to attain more or lessattenuation over a wider range of intensities. Particular surfaces ofreflection could control spectral content of exiting radiation as well.

The device is useful for calibration of lasers and for laser powermeters. It will be understood, however, that its applica tion extends toother radiometric instrumentation where attenuation of radiation isdesirable.

Although the invention has been described with reference to a particularembodiment it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit. and scope of the appended claims.

We claim:

I. In a precision radiation attenuator, an enclosure, a first sphericalbody located in said enclosure having a precision ground reflectingsurface for receiving and dispersing radiation received through anaperture provided therefor in a wall of said enclosure, a secondspherical body located in said enclosure, said second spherical bodybeing mounted for rotation, one portion of said second spherical bodybeing a precision ground reflecting surface for reflecting, dispersingand attenuating radiation received from said first spherical body,another portion of said second spherical body being a black surface forreceiving and absorbing radiation received from said first sphericalbody, an exit aperture in a wall of said en closure for exitingradiation received and reflected from said second spherical body,reflectances of the reflecting spherical bodies and distances betweenthem and also the angles of incidence being known and measurablequantities.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,610,737 Dated 5 October 1971 flw Max Bender and Anthony J. La Rocca Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line #6, matter in parentheses should read --(of the order of10' watts) Signed and sealed this 18th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-IOSO (10-69) uscoMM-oc 60376-P69 U.S. GOVERNMENTPRINTING OFFICI 1 I!" 0-300-834

1. In a precision radiation attenuator, an enclosure, a first sphericalbody located in said enclosure having a precision ground reflectingsurface for receiving and dispersing radiation received through anaperture provided therefor in a wall of said enclosure, a secondspherical body located in said enclosure, said second spherical bodybeing mounted for rotation, one portion of said second spherical bodybeing a precision ground reflecting surface for reflecting, dispersingand attenuating radiation received from said first spherical body,another portion of said second spherical body being a black surface forreceiving and absorbing radiation received from said first sphericalbody, an exit aperture in a wall of said enclosure for exiting radiationreceived and reflected from said second spherical body, reflectances ofthe reflecting spherical bodies and distances between them and also theangles of incidence being known and measurable quantities.